This invention relates to a living body function measuring apparatus, and in particular an apparatus for measuring the function of a periodically changing physiological motion area of a living body, such as the heart and lungs, without inflicting any load or influence on the living tissue and its function.
A variety of methods have been conventionally invented to measure the physiological phenomenon of a living body and reduced to practice. Among the methods an impedance method is known. This method is intended to obtain information on the function of a to-be-measured area of a living body by measuring the electric impedance of the living body area and has the advantages of being capable of measuring the impedance of the living tissue without importing any applicable influence to the living body, capable of a repetitive measurement and capable of a continuous measurement for a lengthy time period.
To explain more in detail the impedance method is intended to measure the function of the heart and lungs, i.e., physiological phenomena on a respiratory or circulatory system, as a variation of electric impedance by permitting a constant slight current of predetermined high frequency to flow through a to-be-measured area of a living body, that is to say, to measure a predetermined living tissue impedance Z.sub.0 and an impedance variation .DELTA.Z which varies by respiration or circulation. Such an impedance method finds the following clinical applications:
(a) the measurement of a cardiac stroke volume PA1 (b) measurement and monitoring of intrathoracic fluid volumes PA1 (c) measurement and monitoring of an inspire/expire pattern, the number of respirations and the relative change of an inspire/expire amount PA1 (d) monitoring of a patient under artificial respiration PA1 (e) measurement of a limb bloodstream
To realize such an impedance method a living body measuring apparatus as shown in FIG. 1 has been put to practice. In FIG. 1, reference numeral 1 is a living body (a subject) to be measured, and 2 is a high frequency wave constant current source for generating a slight current of constant high frequency. 3 and 6 are current applying electrode strips struck to the subject 1. Through the electrode strips 3, 6 a constant slight current is applied from the constant current source 2 to the living body 1. The living body 1 involves a living tissue impedance Z.sub.0 and an impedance .DELTA.Z corresponding to a variation in a physiological motion such as respiration, pulsation etc. A voltage drop due to the presence of such impedances is detected through the voltage detecting electrode strips 4, 5 stuck to the subject 1.
Because an electric current applied to the electrode strips 3, 6 is constant current of predetermined frequency it is possible to obtain a voltage proportional to the impedance of the living body 1 by means of electrode strips 4, 5. Such a voltage is, after amplified at a high-gain AC amplifier 7, supplied to a detector 8 where it is detected and smoothed. The output of the detector 8 becomes a ripple voltage corresponding to a living body impedance between the electrode strips 4 and 5. As shown in FIG. 2, for example, the value of the impedance variation .DELTA.Z is maximal at a maximum inhalation time A in a respiration curve and, since at the end of the expiration phase B the value of .DELTA.Z becomes minimal, the DC and AC components of a ripples voltage delivered from the detector 8 are considered, as shown in FIG. 3, as corresponding to a living tissue impedance Z.sub.0 and impedance variation .DELTA.Z. In consequence, the living tissue impedance Z.sub.0 is considered to be a living body impedance when the value of the impedance variation .DELTA.Z in FIG. 3 is minimal, for example, at the end of the expiration phase B in the respiration curve.
The living tissue impedance Z.sub.0 and impedance variation .DELTA.Z are of importance as parameters for measuring the function of the living body and it is, therefore, necessary to separately obtain from the output of the detector 8 information corresponding to the impedance Z.sub.0 and impedance variation .DELTA.Z. To this end, the output of the detector 8 is delivered to a sample-hold circuit 9 and an external trigger signal is manually applied to the sample-hold circuit 9 in a state in which respiration is stopped at the end of the expiration phase B in FIG. 2 or an external trigger signal is applied to the sample-hold circuit at a time when a maximum expiration level is considered to be attained while observing the output waveform of .DELTA.Z. Thereafter, a voltage E(Z.sub.0) corresponding to the living tissue impedance Z.sub.0 is held in the sample-hold circuit 9. This voltage E(Z.sub.0) is delivered as information Z.sub.0 to a subtraction circuit 10 where it is subtracted from an output voltage E(Z.sub.0 +.DELTA.Z) corresponding to the impedance Z.sub.0 +.DELTA.Z of the detector 8 to obtain a voltage E(.DELTA.Z) corresponding to .DELTA.Z. The voltage corresponding to .DELTA.Z is taken out after amplified at a DC amplifier 11.
Although in the above-mentioned apparatus respiration is stopped at the end of the expiration phase in obtaining a trigger signal, the stopping of respiration imparts a great burden to a patient suffering from a respiratory disease. Furthermore, the subject does not always accurately stop respiration at a maximum expiration time. If respiration is voluntarily stopped, there is a possibility that a data different from that in an ordinary time will be obtained. Even in the method for forming an external trigger signal while observing the waveform of .DELTA.Z, it is difficult to accurately deliver an external trigger signal at a maximum exhalation time. Therefore, a reliable measured data can not obtained due to these reasons and this sometimes leads to a possible erroneous diagnosis.
It is accordingly the object of this invention to provide a living body function measuring apparatus capable of accurately measuring a living body impedance in a normal state without imparting any burden or influence to a living body or its function.
According to this invention there is provided a living body function measuring apparatus comprising means for permitting a slight constant current of high frequency wave of predetermined frequency to flow to a to-be-measured area of a living body, a voltage section for detecting a voltage drop based on the impedance of the living body area, a sample-hold circuit for sampling a predetermined level of the output signal of the voltage detection section and holding the sampled value, means for applying a trigger signal to the sample-hold circuit according to the above-mentioned predetermined level, and means for obtaining a difference output between the output of the voltage detection section and the output of the sample-hold circuit.